The use of high viscosity, low density polyethylene as a coating material for paper products such as milk cartons is well known. During the manufacture of such plastic coated paper products, a variety of virgin scrap material such as roll ends, clippings and off-spec product is produced. This virgin scrap material is distinguished from post-consumer scrap that is subject to a greater degree of contamination.
Because of the relatively high quality of the bleached paper in the virgin scrap material, it is economically advantageous to reclaim it for use in products such as molded paper cups, trays, plates, egg cartons, and the like. Prior to reuse of the virgin scrap material, however, the polyethylene coating is desirably removed. In the first instance, this is done by grinding up the scrap and running it through a hydropulper.
In a hydropulper, the plastic is separated from a major portion of the paper and removed by flotation as an aqueous slurry. The plastic coating waste recovered from a hydropulper can typically comprise, for example, from about 6 to about 33 weight percent paper. The moisture content of the plastic coating waste usually varies in direct proportion to the amount of paper present in the waste because of the hydrophilic nature of paper.
In the past, the plastic coating waste from hydropulpers has been viewed as a discardable waste material. More recently, however, there has been increasing concern over the environmental impact of dumping such polymeric waste. Also, as the price of virgin plastic resins has increased, attention has been given to ways of further cleaning and purifying the plastic resin present in the polymeric waste recovered from hydropulpers.
The use of solvents for recovering polyethylene from a water wet mixture of water-insoluble materials containing a major portion of polyethylene and a minor portion of cellulosic pulp fibers is disclosed, for example, in U.S. Pat. No. 3,226,343.
More recently, a process for producing substantially fiber-free polyethylene pellets from hydropulper plastic coating waste has been disclosed in U.S. Pat. No. 4,332,748. That process discloses the use of tumble drying for dewatering and fiber removal, and also the plasticization of the polyethylene followed by water injection to achieve pellet formation. The polyethylene pellets made by this process comprise less than 5 weight percent fiber, and are preferably fiber-free so that they can be blended off with virgin resin for use in conventional plastic molding and extrusion applications.
In U.S. Pat. No. 5,084,135 to Brooks, et al., issued to assignee, Advanced Environmental Recycling Technologies, Inc., a method for reclaiming plastic and cellulosic fiber from plastic coated waste was disclosed in which high purity plastic pellets comprising about five to 10 weight percent cellulosic fiber, and less than about 10 weight percent water resulted from a process in which plastic coated waste was reduced in size, subjected to flotation with agitation to separate the plastic from the cellulosic fiber. The cellulosic fiber was screened, dried, and baled, the plastic was further reduced in size, and dewatered, then the dewatered plastic was dried and rolled into plastic pellets. This process is effective and commercially advantageous for reducing paper fiber and moisture content of hydropulper plastic coating waste to a desirable level on a cost-effective commercial scale for some applications. However, there continues to be a need for improvement to permit acceptable quality pellets for composites and additional value added uses, such as injection molding and film production.
In particular, the step of separating the polymeric film from a portion of the cellulosic fiber by passing the plastic waste through at least one wash tank and further, the step of separating the polymeric film and a further portion of the cellulosic fiber by flotation with agitation, both resulted in significant contamination of the washwater and flotation water with cellulosic fibers and other contaminants. Filtration of the washwater and the flotation water for purposes of re-use and/or for purposes of environmentally safe discharge of the water was required.
Further, it has been found that the pelletizing step of the previous issued co-owned U.S. Pat. No. 5,084,135 could result in inconsistent plastic pellets due to either inadequate frictional heat buildup for economic densification during pelletization, or excessive heat buildup during pelletization and potential thermal degradation of the plastic. This process was monitored by experienced technicians on a substantially continuous basis in order to observe the pellet output and manually regulate the temperature of the pelletizer and thereby control the consistency of the rolled pellets.
Notwithstanding the processes disclosed in the prior art, however, an improved method is needed for reducing the contaminants, paper fiber, and moisture content of hydropulper plastic coating waste or polymeric film to a desirable level on a cost effective, commercial scale. More specifically, an economical and reliable method is needed for reducing the paper and moisture contents of such plastic coating waste to acceptable levels, preferably from about 5 to about 10 weight percent cellulosic fiber and less than about 10 weight water for composites, and less for film and injection molding, and for pelletizing the resultant material.
In recent years the use of plastic film grocery bags and shopping bags, often made of polymeric material such as low density polyethylene, linear low density polyethylene, and high density polyethylene, has become widespread. To alleviate possible adverse environmental impact from the disposal of such bags in landfills, efforts are now made to recycle such bags. However, bags returned by consumers frequently contain paper scraps such as sales receipts, coupons, labels, adhesive stickers, organic materials, and food waste. Problems have been encountered in trying to recover and reuse polymeric film from such postconsumer waste in place of virgin resin.
Another source of recycled polymeric film is the shrinkstretch film used in wrapping food products such as produce, meats, bakery goods, and the like. This recycled film is often mixed with organic materials or with dirt accumulated when bales are stacked or warehoused, or with paper scraps such as, for example, cash register receipts, coupons, paper labels or price stickers that are attached or adhered to the polymeric film, and an effective method of recovering and reusing such recycled scrap for the same or for different purposes is also needed.